If its a combi boiler problem will almost definately be cause by either dirt or scale(depending on water hardness) in the plate heat exchanger. Best advice if this is the problem is a replacement dont get fobbed with a clean it will be just as bad in a very short time.
SE gas man
February 2009
Automatic Answer
We may have found other answers which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.